Pure benzene can be obtained particularly by solvent extraction or by extractive distillation of the top effluents from naphtha-pre-fractionating columns before reforming or from columns used for post-fractionating catalytic reforming effluents.
It is known that, to reduce the benzene content of fuels, refiners use pre-fractionation of naphthas before reforming or post-fractionation of reforming effluents. Pre-fractionation makes it possible to eliminate a large portion of the benzene and its precursors (paraffins and naphthenes) at the top of the distillation and to obtain, at the bottom, a C.sub.7+ fraction that feeds the reforming. Post-fractionation also makes it possible to obtain at the top of the column a light benzene-rich fraction and, at the bottom, a C.sub.7+ fraction that feeds the fuel pool.
Benzene is present in these top effluents at relatively low concentrations, for example, 0 to 5% in the pre-fractionating elements and 1 to 10% in the post-fractionating effluents. Because of the small difference in volatility between these hydrocarbon batches and the other components of C.sub.5 to C.sub.7, it is difficult to separate benzene by simple distillation between the light components at the top and the heavy components at the bottom.
Only processes that use a selective solvent make it possible to obtain benzene with a purity of 99.9% by weight, which is required if it is to be used as a petrochemical base.
Thus, the processes most commonly used for purification of benzene are extractive distillation and solvent extraction. The difference between the two processes depends on the physico-chemical characteristics of the solvent that is used relative to those of benzene and consequently the method for regenerating the solvent.
These different treatments are disadvantaged, however, because of the low benzene content of the effluents that are obtained at the top of pre-fractionating columns or post-fractionating columns, which necessitates the use of large quantities of solvent.
One way of obtaining an effluent that is more concentrated in benzene and, consequently, of reducing the flow of solvent optionally required to extract the benzene is to carry out lateral drawing-off on the pre-fractionating column or post-fractionating column at the level where benzene concentration exhibits a peak. The benzene contents of such drawing-off do not exceed 10 to 30%, however.